Fundamentally, when boring underground, there is a need to guide the underground boring apparatus to a specific destination, or to move it on a desired track. Particularly when boring horizontally, the directional accuracy of the underground boring apparatus, which in some cases is used in particular underground in densely built-up regions with a complex infrastructure, plays a major role. Firstly, the underground boring apparatus must be able to accurately reach a destination trench, which is frequently tightly constrained, in order to carry a pipe or a cable to a desired position, or in order to make it possible to emerge from the earth's surface at a specific point. Secondly, uncontrolled deviation of the underground boring apparatus from the nominal boring axis can lead to damage to underground pipelines or fittings.
In the last 20 years, numerous underground boring apparatuses and methods for such appliances have therefore been developed in the prior art, in order to steer an underground boring apparatus of this type as accurately as possible, or to move it in a straight line as reliably as possible.
The majority of the underground boring apparatuses which have been developed either operate on the principle that the boring apparatus has a forward-drive head which can be moved from a central or symmetrical position to an asymmetric position in order to initiate a turn for the underground boring apparatus (Group I), or they have an asymmetric forward-drive head, with the forward-drive head or the underground boring apparatus being rotated continuously when moving in a straight line, and with the rotation being interrupted at a specific angular position in order to initiate a turn (Group II).
A Group I appliance, that is to say an appliance with an adjustable forward-drive head, is described, for example, in DE 37 35 018 A1. Appliances such as these have a mechanism in order either to pivot a forward-drive head (which is symmetrical in the initial position) out of the symmetrical position for example by using an eccentric ring or moving an eccentrically mounted symmetrical forward-drive head by rotation from the boring axis relative to the appliance. In all cases, the mechanism (which can be operated mechanically or hydraulically) leads to the appliance being moved from a symmetrical “straight ahead” basic arrangement to an asymmetric “turning” arrangement. It is necessary to know the relative position of the forward-drive head with respect to the underground boring apparatus in order to control these appliances. This relative position can be transmitted by means of appropriate sensors to an operator, who can then use further measurement and display devices to determine the position of the underground boring apparatus, and to change the movement direction.
In Group II steerable underground boring apparatuses, such as those described, for example, in U.S. Pat. No. 4,907,658, are permanently asymmetric, for example inclined, forward-drive head leads to a continuous steering movement of the underground boring apparatus when it is being driven forward. In order to move straight ahead, the underground boring apparatus or the forward-drive head is caused to rotate, which leads to a tumbling boring movement of the appliance, running essentially straight ahead. In order to maintain the continuous rotation of the head or of the appliance, U.S. Pat. No. 4,694,913 provides a mechanical device which is arranged outside the borehole and uses a linkage to rotate the underground boring apparatus. Although this apparatus and procedure allow the underground boring apparatus to be controlled to a certain extent, they involve considerable design and mechanical complexity since, in addition to the system for driving the underground boring apparatus forward, a system must be provided for rotation, and a linkage must be provided for transmitting the rotation. The linkages are relatively rigid, impede the steering process, and cannot be moved from the trench to the surface. In some appliances such as these, a constricted flexible linkage is used as the first linkage behind the underground boring apparatus in order to allow the underground boring apparatus to carry out any steering movements whatsoever.
Other underground boring apparatuses avoid this complexity for moving in a straight line by using their own systems for rotating the underground boring apparatus or the forward-drive head, as is described, by way of example, in DE 39 11 467 A1.
In all cases, the advantage of the steerability of an underground boring apparatus leads to not inconsiderable use of materials, costs and operating complexity.
In addition to horizontal boring methods using underground boring apparatuses, boring methods and apparatuses are also known in which a linkage which is provided with a forward-drive head is introduced into the ground via a forward-drive unit which acts outside the borehole, as is described, by way of example, in DE 92 07 047 U1. In this method, the linkage which is required for controlling or rotating the asymmetric forward-drive head is already provided for the forward-drive and is used for rotation, so that the problem of rotation when moving underground boring apparatuses straight ahead does not arise. However, extensive equipment must be provided for each borehole for this purpose and, in particular, must be transported (linkage, drive), thus increasing the amount of effort and the costs.
PCT-WO 94/05941, to which reference is hereby made, attempts to solve the problem of steering an underground boring apparatus by means of a Group I appliance, in which the forward-drive head can be moved from a symmetrical position (boring in a straight line) to an asymmetric arrangement (turning) by rotation relative to the underground boring apparatus. In this appliance, the forward-drive head is in the form of a cone with guide plates which extend from the tip of the head along the forward drive axis, in order to fix the head when driving forward and during steering, and has a longitudinal axis which is inclined to the forward drive axis of the appliance. The forward-drive head has a rear contact surface, which makes contact with a front contact surface on the underground boring apparatus, and on which the forward-drive head is rotated. The plane of these contact surfaces is inclined to the appliance longitudinal axis. This makes it possible to rotate the appliance casing about its longitudinal axis, while the earth holds the forward-drive head firmly by means of the guide plates.
Such casing rotation allows the forward-drive head to be moved to an eccentric position with respect to the appliance casing, in which position it will turn.
The rotation angle—referred to as the difference angle in the following text—between the forward-drive head and the appliance casing or the two limit positions of the forward-drive head is governed by a driver pin, which is connected to the forward-drive head and engages in a circular slot in the appliance casing. When the pin is located at one end of the appliance slot, then the forward-drive head is in its position for moving in a straight line (straight-ahead position), while, at the other end of the appliance slot, it is located in the position for turning (steering position).
In order to move the underground boring apparatus from moving in a straight line onto a specific turning track, the appliance casing can be rotated by means of the compressed-air hose sufficiently for the appliance to achieve the desired angular position (initial position) for the desired turning track.
This rotational movement may be composed of two phases. In this case, the first phase is for only the appliance casing to be rotated first of all, until the driver pin has moved through the entire difference angle from the straight-ahead position to the steering position. As soon as this has been done, the forward-drive head and the appliance casing are coupled to one another for the rest of the rotational movement, that is to say the appliance casing and the forward-drive head rotate together until the initial position for turning is reached. In the process, considerable forces must be applied, since the deflection of the forward drive head causes a lateral movement of the surrounding earth and the forward-drive head has to move the earth by means of its guide plates during the rotation process. The force required is exacerbated by the fact that the earth has been compressed by the bore face of the forward drive head.
It is necessary to determine, above the ground, the angular position, with respect to the appliance longitudinal axis, in which the steering position is located, that is to say that end of the casing slot which governs turning.
If, for example, the steering position is in the 6 o'clock position when boring in a straight line and it is intended to move the underground boring apparatus from this position to a curved track running upward in a vertical plane, then the steering position is changed to the 12 o'clock position. This is done by using compressed air run through a compressed-air hose to rotate the appliance casing. If the forward-drive head or its driver pin is in the straight-ahead position, then the appliance casing is first rotated through the difference angle on its own until the driver pin is located at the other slot end in the steering position, and then the casing is rotated, together with the forward-drive head which is now in the steering position, to the 12 o'clock position.
Since the steering head position is unknown outside the borehole, it is also impossible to find out how much hose rotation is required—with or without overcoming the difference angle—to move the steering position to the correct initial position for turning.
Owing to these difficulties, the appliance described above has been developed in DE 199 10 292 A1, to which reference is hereby made, such that, when changing from movement in a straight line to turning, the forward-drive head position at that time with respect to the appliance casing and the position of the driver pin in the casing slot are determined first of all, and the appliance together with the forward-drive head is then set to the desired turning track, or is moved to the initial position for turning, by rotating the compressed-air hose.
However, the problem still remains that the forward drive head is fixed in the highly compressed earth surrounding it. It is virtually impossible to use the appliance described in PCT WO 94/05941 in practice, since the forces required to rotate the underground boring apparatus and the forward drive head are so large that rotation is frequently impossible or leads to kinking of the compressed air hose. Furthermore, the guide plates on the forward-drive head often result in uncontrolled deflection of the appliance in an undesirable direction, for example due to stones or the like.
DE 199 47 645 A1 describes a process in which the underground boring apparatus is rotated manually between two or more positions on a path section basis via the supply line, that is to say discontinuously via the supply line, with the forward-drive head being permanently arranged asymmetrically.
For turning, the underground boring apparatus is driven forward without any change in its angular position over a specific path section. For moving in a straight line, the path section-by-path section oscillating movement makes it possible to change between the 12 o'clock position and the 6 o'clock position. When moving in a straight line, this leads to an oscillating movement in the vertical axis.
This method admittedly makes steering easier, since the forward drive head is no longer rotated relative to the underground boring apparatus and, in consequence, need not have any guide plates. However, when moving straight ahead, it is necessary to carry out a continuous to and fro steering movement by using compressed-air through the compressed air hose, which in some circumstances leads to undesirable control effort.